| Title | Fluid Geochemistry Characteristics of Simbolon and Pusuk Bukit Geothermal Area in Toba Caldera, North Sumatra Province, Indonesia |
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| Authors | Brenda Ariesty Kusumasari, Kotaro Yonezu, Niniek Rina Herdianita, Sachihiro Taguchi, Koichiro Watanabe |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | geochemistry, Lake Toba, caldera, water chemistry, gas chemistry, stable isotope, steam-heated |
| Abstract | Lake Toba is administratively located in North Sumatra Province, Indonesia. Lake Toba is known as the largest volcanic lake on the earth formed by an explosive eruption about 74 ka. On the surrounding area of Lake Toba, hot springs are largely found. These hot springs are accompanied by alteration on the surrounding area. The most extensive hot springs are located on Pusuk Bukit and Simbolon area on the western part of the lake. Preliminary study of the geothermal system on Pusuk Bukit and Simbolon area revealed that these two areas are the hydrothermal system associated with Toba Caldera. Geochemical analysis from hot springs provides information about the occurrence of two high temperature systems. The first system is in Pusuk Bukit which is associated with young volcanism. The up-flow in this area yields acid and sulfate-type hot springs. The gas chemistry shows that the source of the gas comes from meteoric origin. The second system namely Simbolon is associated with active heat source beneath Lake Toba (estimated as the remaining heat after the formation of Toba Caldera). Hot springs on Simbolon are mostly bicarbonate and sulfate water with near neutral acidity. Fluids from Pusuk Bukit and Simbolon are the up-flow from each system. The fluids from both areas are immature fluid therefore solute geothermometer could not be applied. The fluids have high content of Na, K, and Mg. The geothermal fluid has also sustained water-rock interaction that is also supported by the occurrence of highly altered rocks largely found on the surrounding area. The stable isotope data of ä18O and äD indicates boiling process for fluid in Simbolon. While fluid in Pusuk Bukit undergoes boiling and evaporation process. Based on geoindicator and stable isotope data, the geothermal fluid has meteoric origin. |